Prior art FIG. 1 is a top down view of an LED die 10, and FIG. 2 is a simplified cross-sectional view of the LED 10 along line 2-2 in FIG. 1. In the example, the LED die 10 is GaN based and has its growth substrate removed. The structure is well known. A bottom metal anode electrode 12 is typically bonded directly to a submount pad or to a circuit board. A metal reflector 14 over the electrode 12 reflects light upward. The LED's epitaxially-grown semiconductor layers include a first p-type layer 16, a p-type cladding layer 18, an active layer 20, an n-type cladding layer 22, a first n-type layer 24, and a second n-type layer 26. The various p and n-type layers that interface between the cladding layers and the metal contacts may have different doping amounts and different compositions to achieve different functions such as lattice matching and current spreading. There may be many more layers. The semiconductor layers are transparent.
A transparent current spreading layer 28 is formed over the second n-type layer 26, and a metal cathode electrode 30 is electrically connected to an edge of the current spreading layer 28. A wire (not shown) is bonded to the cathode electrode 30. The current-spreading layer material is selected for low optical loss, low resistivity, and good electrical contact. Suitable materials for the current-spreading layer 28 include are Indium Tin Oxide, Zinc Oxide, or other transparent conducting oxides. The current spreading layer 28 is only a few microns thick so has a low vertical resistance and a much higher lateral resistance. It is important that the current distribution over the p-type cladding layer 18 and n-type cladding layer 22 is fairly uniform to achieve uniform light generation across the active layer 20.
To compensate for the relatively high lateral resistance of the current spreading layer 28, a low-resistance metal shunting layer 32 is patterned to extend across the current spreading layer 28 yet block only a small amount of light. There is a tradeoff between minimizing current crowding and minimizing light blockage. The shunting pattern shown in FIG. 1 is typical, with metal bus bars along the periphery of the die 10 and perpendicular metal bus bars connecting them. These shunting strips are formed very narrow to minimize light blockage.
FIG. 2 shows the current flow through the LED die 10 with thick arrows 36 and some photon trajectories with thin arrows 38. A simplified emitted light pattern 39 is also shown.
The top surface of the LED die 10 is roughened to increase light extraction.
One problem with the conventional shunting designs is that the thin shunting strips exhibit a contact resistance at the interface of the strips and the current spreading layer 28, where the contact resistance is directly related to the width of the strips.
For the particular case of a patterned shunting layer characterized by bus bars as shown in FIG. 1, the contact resistance of one of the three inner crossing bus bars may be expressed as,
                              R                      C            ⁡                          (              ric              )                                      =                                            R              s                                      2              ⁢              L                                ⁢                                    L              t                        ·                          coth              ⁡                              (                                  w                                      2                    ⁢                                          L                      t                                                                      )                                                                        eq        .                                  ⁢        1            
where resistance Rs is the sheet resistance (in Ω/□) of the current spreading layer 28, L is the length of the bus bar section, w is the width of the bus bar, and Lt is the transfer length, expressed in unit length. The transfer length is defined as,
                              L          t                =                                            ρ              c                                      R              s                                                          eq        .                                  ⁢        2            
where ρc is the contact resistivity of the metal-semiconductor interface, expressed in Ω·m2.
As is well known, lateral current between a conductive layer and a metal contact is not uniform across the contact. The voltage is highest near the edge of the contact and drops substantially exponentially with distance. The 1/e distance of the voltage curve is another way to determine transfer length.
FIG. 3 represents the above contact resistance expression normalized against Rs as function of the normalized quantity w/Lt for the case of L=Lt. The curve indicates that for contact widths smaller than 2Lt the contact resistance increases inversely proportional to w, as
      R          C      ⁡              (        ric        )              →                    R        s            wL        ⁢                  L        t        2            .      On the other hand, for contact widths higher than 2Lt the contact resistance approaches the quantity
            R      s              2      ⁢      L        ⁢      L    t  as
  coth  ⁡      (          w              2        ⁢                  L          t                      )  tends to 1.
As seen, the widths of the bus bars in FIG. 1 cannot be made too small or else the contact resistance will be too high, yet narrow widths are desirable to block less light.
Therefore, it would be desirable to reduce the contact resistance between a metal shunting layer and the current spreading layer without adversely impacting the light extraction of the LED die. Conversely, it would be desirable to increase the light extraction of the LED die without reducing the contact resistance between a metal shunting layer and the current spreading layer. It is also desirable to improve the current distribution uniformity across the surface of the LED die.